docs: def attached_window(self):
          """
          Return active :class:`Window` object.
  
          Returns
          -------
          :class:`Window`
          """
          active_windows = []
          for window in self._windows:
              if 'window_active' in window:
                  # for now window_active is a unicode
                  if window.get('window_active') == '1':
                      active_windows.append(Window(session=self, **window))
                  else:
                      continue
  
          if len(active_windows) == int(1):
              return active_windows[0]
          else:
              raise exc.LibTmuxException(
                  'multiple active windows found. %s' % active_windows
              )
  
          if len(self._windows) == int(0):
              raise exc.LibTmuxException('No Windows')